Colour, smell and taste – the beautiful world of plants and

chemistry

Alison Foster, University of Oxford Botanic Garden

Dubrovnik, Croatia, 2012

Plants and Chemistry• Colour• Smell• Taste

Colour

• Main pigment types

– Porphyrins (nitrogen containing)

– Flavonoids (incl. anthocyanins)

– Carotenoids (terpenes)

– Betalains (nitrogen containing)

Colour

O+HO

OH

OH

OH

O Glucose

β-carotene

cyanidin-3-glucoside

N+

HO

HO

H

COO-

NH

CO2HHO2CH

betanidin

ColourO+HO

OH

OH

OH

O Glucose

Anthocyanins:

• Found in many plant families – except those in Caryophyllales

• Found in all plant tissues• 109 tonnes produced per year in

nature• 2% of all carbon fixed via

photosynthesis is converted to anthocyanins

Ferns (and Fern allies)

Gymnosperms

Angiosperms

ColourO+HO

OH

OH

OH

O Glucose

Functions of anthocyanins:

• Pollinator attractant

• Sunscreen – succulents and autumn leaf colouration

• Anti-feedants – bitter tasting

Colour

Anthocyanins as pollinator attractant?

Bromeliaceae

Colour

Anthocyanins as sunscreen or antifeedant or both?

Colour

Anthocyanins as antifeedant ?

Colour

Metasequoia glyptostroboides

dawn redwood

Rhodoxanthin:

Colour

Daucus carrota

carrot

beta-carotene

Colour

Solanum lycopersicon

tomato

lycopene:

Colour

Betalains:

• Amino-acid derived• Nitrogen containing – metabolically more expensive

• Yellow to red pigments (betaxanthins and betacyanins)

• Only present in plants in order Caryophyllales

N+

HO

HO

H

COO-

NH

CO2HHO2C

H

Caryophyllales

Colour

Betalains:

• Caryophyllales:• Beetroot (Amaranthaceae)

• Bougainvillea (Nyctaginaceae)

• Pokeweed (Phytolaccaceae)

• Carnations (Caryophyllaceae)

N+

HO

HO

H

COO-

NH

CO2HHO2C

H

Smell

• Nicotiana attenuata from the SW United States– Releases benzyl acetone

when flower opens at night– Attracts hawkmoth pollinators

• Pollinators are also herbivores (larval stage)

• Upon herbivore attack, flowers no longer produce benzyl acetone nor open at night

• Day opening flowers are then hummingbird pollinated

O

Smell• Lavandula

angustifolia

camphor cineole

linalyl acetate

linalool

Smell• Amorphophallus titanum• Spadix emits smell of

rotting flesh• Pollinators are attracted

– Carrion-eating beetles– Flesh flies

• Smell made up of amines and disulfides

H2NNH2

H2N NH2

putrescine

cadaverine

Smell

• Cercidiphyllum japonicum– Senescing and abscising leaves– Green leaves contain maltol glucosides– Yellow brown/fallen leaves contain maltol

O

O

OH

Taste• Chilli peppers

(Capsicum species) contain capsaicin

• “Hotness” measured on the Scoville scale

• Test method devised in 1912 by Wilbur Scoville

• Now measured by HPLC

HO

OMe

NH

O

Taste

0

• Jalapeño pepper

• Scotch Bonnet

5,000

100,000

• Naga jolokia 1,040,000

• Sweet pepper

Chilli pepper Scoville Heat Units

Taste

• Euphorbia resiniferatoxin

•Resiniferatoxin

O

O

O

OH

OMe

OO

O

H

HO

16 Billion SHU

Taste

• Ginger – Zingiber officinalis, Zingiberaceae

• Active constituent of fresh ginger is gingerol

• Cooking ginger transforms it into zingerone

HO

OMe

O OH

HO

OMe

O

Taste• Ginger – Zingiber

officinalis, Zingiberaceae

• Dried ginger contains more of another molecule, shogaol

• The Japanese name for ginger is shōga

HO

OMe

O OH

Taste• Ginger – Zingiber

officinalis, Zingiberaceae

• Dried ginger contains more of another molecule, shogaol

• The Japanese name for ginger is shōga

HO

OMe

O OH

HO

OMe

O

Taste• Vanilla– Vanilla

planifolia, Orchidaceae

Taste

HO

OMe

NH

O

HO

OMe

O

HO

OMe

O OH

O

O

O

OH

OMe

OO

O

H

HO

Taste

caffeine

theophylline theobromine

Taste

theobromine caffeinetheophylline

Conclusions